In the last decade, antennas constructed using printed circuit techniques have become popular especially for mobile applications. These antennas are often thin and can be affixed to a vehicle, aircraft, etc. without appreciably altering the aerodynamics of the host structure.
The printed circuit antennas of the prior art are often of the resonant type. In such antennas, the input impedance varies widely with a change of energizing frequency, which frequency is in the vicinity of the frequency of resonance. This thereby severly limits the antenna's operating bandwidth typically limiting it to only a few percent of the resonant frequency.
To overcome these limitations, others have constructed non resonant, travelling wave printed circuit antennas from microstrip lines. For instance, see "Curved Microstrip Lines as Compact Wideband Circularly Polarized Antennas" by C. Wood, IEE Journal, Microwaves, Optics and Acoustics, January 1979, Volume 3, No. 1, Pages 5-13. Wood describes various antennas with conducting strip geometry of both constant and varying width overlying closely spaced flat ground planes. Wood's antennas develop their radiating field between the plane of the microstrip and the ground plane and radiate circularly polarized waves. The method used by Wood to excite his antennas is to attach the center conductor of a coaxial cable to the conducting strip at some location on the strip and to connect the outer conductor to the ground plane. As a consequence there is an abrupt change in geometry at the connection which has a deleterious effect on the input impedance as a function of frequency, thus limiting the operating bandwidth.
It can be shown that the characteristic impedance of a strip line conductor is a function of the ratio of the width of the strip to its height above the ground surface. Thus, if the width of a strip conductor varies substantially along its length while maintaining a constant height over the ground plane, its characteristic impedance may vary in an unacceptable manner. On the other hand, if the aforementioned width to the height ratio remains constant, then the characteristic impedance of the antenna structure remains essentially constant as the wave moves along the structure.
Accordingly, it is an object of this invention to provide a microstrip antenna which exhibits an impedance characteristic that remains nearly constant over a wide band of applied frequencies.
It is a further object of this invention to provide a microstrip antenna of thin dimension capable of being mounted on vehicles and other moving conveyances.
It is a further object of this invention to provide a curved microstrip antenna of thin configuration which is adapted to both transmit and receive signals of a circular polarization.